As Aesop said, appearances are deceiving—even in life's tiniest
critters. From first detection in the 1880s, clinging to the sides of
an aquarium, to its recent characterization by the U.S. Department of
Energy Joint Genome Institute (DOE JGI), a simple and primitive animal,
Trichoplax adhaerens, appears to harbor a far more complex suite of
capabilities than meets the eye. The findings, reported in the August
21 online edition of the journal Nature, establish a group of
organisms as a branching point of animal evolution and identify sets of
genes, or a "parts list," employed by organisms that have evolved along
particular branches.

With each sequenced genome, another dataset is made available to
advance the quest of evolutionary biologists seeking to reconstruct the
tree of life. The analysis of the 98 million base pair genome of
Trichoplax (literally "hairy-plate") illuminates its ancestral
relationship to other animals. Trichoplax is the sole member of the
placozoan ("tablet," or "flat" animal) phylum, whose relationship to
other animals, such as bilaterians (humans, flies, worms, snails, et
al) and cnidarians (jellyfish, sea anemones, corals, et al), and
sponges is contentious.

"Our whole genome analysis supports placing the placozoans after
the sponge lineage branched from other animals," said Daniel Rokhsar,
the publication's senior author, DOE JGI's head of Computational
Genomics Program, and Professor of Genetics, Genomics and Development
at the University of California, Berkeley.

"Trichoplax has had just as much time to evolve as humans, but
because of its morphological simplicity, it is tempting to think of it
as a surrogate for an early animal," said Mansi Srivastava, the study's
first author, a graduate student under the direction of Rokhsar, at the
Center for Integrative Genomics, U.C. Berkeley.

Earlier mitochondrial DNA studies suggested that this "mother of all
metazoans," Trichoplax, was the earliest branch, before sponges
diverged, but this remains debatable—even among collaborators.
__IMAGE_3

"The latest and most complex analysis again suggests that placozoans
populated the oceans long before sponges evolved," said Bernd
Schierwater, director of the Institute of Animal Ecology & Cell
Biology and head of the Center for Biodiversity at TiHo Hannover,
Germany. Schierwater, a study co-author, joined Stephen Dellaporta and
Leo Buss of Yale University in proposing the Trichoplax sequencing
project in 2004 to DOE JGI's Community Sequencing Program [http://www.jgi.doe.gov/CSP/overview.html].

"The outcome of the Trichoplax adhaerens genome sequencing is so
exciting that we are now culturing another 13 placozoan species in
order to identify the most basal placozoan lineage and genome," said
Schierwater.

"Trichoplax is an ancient lineage—a good representation of the
ancestral genome that is shedding light of the kinds of genes, the
structures of genes, and even how these genes were arranged on the
genome in the common ancestor 600 million years ago," said Srivastava.
"It has retained a lot of primitive features relative to other living
animals."

Originally collected from the Red Sea, and cultured over the last 40
years in the laboratory, Trichoplax is a two-millimeter flat disk
containing fluid sandwiched between two cell layers. It lacks organs
and only has four or five cell types. Yet, despite its apparent
simplicity, its genome encodes a panoply of signaling genes and
transcription factors usually associated with more complex animals.

Trichoplax has no neurons, but has many genes that are associated
with neural function in more complex animals. "It lacks a nervous
system, but it still is able to respond to environmental stimuli. "It
has genes, such as ion channels and receptors, that we associate with
neuronal functions, but no neurons have ever been reported," explained
Rokhsar.

Of the 11,514 genes identified in the six chromosomes of Trichoplax,
80 percent are shared with cnidarians and bilaterians. Trichoplax also
shares over 80 percent of its introns—the regions within genes that are
not translated into proteins—with humans. Even the arrangement of genes
is conserved between the Trichoplax and human genomes. This stands in
contrast to other model systems such as fruit flies and soil nematodes
that have experienced a paring down of non-coding regions and a loss of
the ancestral genome organizations.

With its pancake shape, gutless feeding, and genomic primitiveness,
the rich array of metabolic capabilities begs additional consideration.
While it has been observed to motor around via cilia, eat by mounting
its prey, and reproduce by fission (pulling itself into pieces)—it may
in fact have a secret sex life.

"Some of our new placozoan species show frequent sexual reproduction
while others never show any signs of sex," said Schierwater. "The
genome data allow us to search for the genes responsible for sex and
life cycle complexity."

"It's remarkable that we have the whole genome sequence but we still
know so little about this animal in the wild," said Rokhsar. "Hopefully
the genome sequence will stimulate more studies of this enigmatic
creature."